From 65e072bae9130ee286028bbffaff211a9cee1eba Mon Sep 17 00:00:00 2001
From: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Date: Tue, 24 Feb 2026 00:53:17 +0000
Subject: [PATCH] `_struct.c`: Fix UB from integer overflow in `prepare_s`
 (GH-145158)

Avoid possible undefined behaviour from signed overflow in `struct` module

As discovered via oss-fuzz.
(cherry picked from commit fd0400585eb957c7d10812d87a8cb9e1f3c72519)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
---
 Lib/test/test_struct.py                                |  3 +++
 .../2026-02-23-20-52-55.gh-issue-145158.vWJtxI.rst     |  2 ++
 Modules/_struct.c                                      | 10 +++++++++-
 3 files changed, 14 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-02-23-20-52-55.gh-issue-145158.vWJtxI.rst

diff --git a/Lib/test/test_struct.py b/Lib/test/test_struct.py
index 6b71df4b1f9559..e602eeb6289808 100644
--- a/Lib/test/test_struct.py
+++ b/Lib/test/test_struct.py
@@ -547,6 +547,9 @@ def test_count_overflow(self):
         hugecount2 = '{}b{}H'.format(sys.maxsize//2, sys.maxsize//2)
         self.assertRaises(struct.error, struct.calcsize, hugecount2)
 
+        hugecount3 = '{}i{}q'.format(sys.maxsize // 4, sys.maxsize // 8)
+        self.assertRaises(struct.error, struct.calcsize, hugecount3)
+
     def test_trailing_counter(self):
         store = array.array('b', b' '*100)
 
diff --git a/Misc/NEWS.d/next/Library/2026-02-23-20-52-55.gh-issue-145158.vWJtxI.rst b/Misc/NEWS.d/next/Library/2026-02-23-20-52-55.gh-issue-145158.vWJtxI.rst
new file mode 100644
index 00000000000000..60a5e4ad1f0ca4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-02-23-20-52-55.gh-issue-145158.vWJtxI.rst
@@ -0,0 +1,2 @@
+Avoid undefined behaviour from signed integer overflow when parsing format
+strings in the :mod:`struct` module.
diff --git a/Modules/_struct.c b/Modules/_struct.c
index 016a1d3cf21231..55b32f074779b7 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1478,7 +1478,15 @@ prepare_s(PyStructObject *self)
             case 's': /* fall through */
             case 'p': len++; ncodes++; break;
             case 'x': break;
-            default: len += num; if (num) ncodes++; break;
+            default:
+                if (num > PY_SSIZE_T_MAX - len) {
+                    goto overflow;
+                }
+                len += num;
+                if (num) {
+                    ncodes++;
+                }
+                break;
         }
 
         itemsize = e->size;